On helicopter-supporting rotors, whose hinged-type hubs comprise, in particular, for each blade what is termed a drag hinge articulated around an axis perpendicular to the plane of the blade, thereby allowing this blade to freely oscillate within the rotational plane of the rotor, it is well understood that advantage is gained by providing for, around this hinge, the vigorous elastic return of each blade toward its median angular position, in such a way as to be able to adjust the natural vibrating frequency of the blade assembly on which drag is exerted, and it is also well understood that it is imperative to dampen these angular drag oscillations of the blade in order to minimize the transmission of dynamic drag stresses traveling from the blade toward the helicopter frame, while avoiding any instability which may arise from connections between oscillations resulting from aerodynamic stresses and the mechanical reactions of rotor and fuselage elements, both during the operational raising and lowering of the rotor speed durIng take-off and landing, and in flight when the rotor turns with its substantially stabilized operational rotation speed.
It is known that the dynamic drag characteristics of a blade of a helicopter-supporting rotor are a function of the value of its initial natural mode of vibration under the force of drag, which is in turn a function, first, of the centrifugal return of the blade on which drag occurs, caused by the centrifugal force which is exerted radially to the center of gravity of the blade, and of a possible additional elastic drag-induced return brought about by any device exerting elastic resistance to the angular displacements of the blade around its median position under the force of drag, and, second, of damping induced in the alternating angular movements of the blade around its drag hinge.
To obtain the correct operation of the rotor in the face of these dynamic phenomena, the two parameters, i.e., the position of the initial drag mode and damping, must be mutually adjusted.
To meet this requirement, it has recently been proposed that resilient return struts of the hydraulic-elastic type having incorporated damping be used, each of which is a piece of equipment providing simultaneously for the elastic return of a blade subjected to drag by means of the deformation of a mass of flexible material, and damping by means of a hydraulic device.
Struts of this type, examples of which are described in French Patents Nos. 2 528 382 and 2 592 696, have been developed based on rotating dampers operating by throttling and using high-viscosity fluids, such as those described in French Patents Nos. 2 256 347 and 2 592 449, as well as tubular fluid dampers such as those described in French Patent No. 2 573 829, whose principles of operation have been proposed as models for the construction of resilient return struts having incorporated linear damping.
The rotational damper operating by throttling a high-viscosity fluid, as described in French Patent 2 256 347, is composed of the coaxial arrangement of a rotor and a stator fitted with radial blades which are made unitary alternately with the rotor and the stator in a circumferential direction, and demarcating within the damper, which is closed off transversely from the axis of the rotor and the stator by at least one cover, a main inner chamber filled with a viscous fluid.
The blades are connected to means for the throttling of the fluid which flows from one to the other of different capacities set off by the blades within the main inner chamber when the damper rotor and stator are induced in relative rotation around their common axis. Damping is thus produced by the transfer of the viscous fluid through the throttling means produced by the action of the rotation of the blades connected to the rotor, in relation to the blades connected to the stator. Compensation for thermal expansion of the viscous fluid is provided for by the existence of an auxiliary chamber filled with a volume of this viscous fluid and permanently feeding into the main inner chamber by at least one small-diameter hole opened in a bottom of the damper separating the main and auxiliary chambers. The viscous fluid contained in the auxiliary chamber is pressurized by a piston activated by spring washers, in order to ensure the constant feeding of the main chamber under static pressure. Furthermore, the auxiliary chamber is connected to the main chamber by at least one hole having a larger diameter which cooperates with a supercharging valve equipped with a spring-leaf attached to the bottom surface through which the large-diameter hole is drilled on the side facing the inner chamber, in such a way as to act as a reverse-lock valve which inhibits the flow of the viscous fluid from the main chamber toward the auxiliary chamber, but which permits a flow in the opposite direction when the part of the main chamber in which this large-diameter hole opens out is depressed.
A rotational damper of this kind is made to be particularly economical because of the use of a high-viscosity fluid, which makes it possible to attribute high values to the operational play among the functioning parts of the damper, and simultaneously to use simplified throttle means to create the damping forces, as well as valves having an equally simple structure for related functions, such as compensation for thermal expansion of the viscous fluid and the supercharging of the main chamber of the damper.
A rotating damper of this kind was perfected in French Patent No. 2 592 449, according to which the auxiliary chamber is formed at least partially by a diaphragm which is elastically deformable, thus making it possible to compensate for variations in the volume of the fluid as a function of its temperature. An elastically-deformable diaphragm is, advantageously, fitted on each end, and substantially transverse, surface of the damper, in such a way that two auxiliary chambers are thus demarcated on each side of the stator and rotor, respectively, and, more precisely, formed between one end surface of the damper and the corresponding diaphragm, and at least one feeding passage between each auxiliary chamber and the main chamber is fitted in the corresponding end surface, this feeding passage being small enough to act as a dynamic filter for the frequencies of use of the damper. This arrangement makes it possible to limit the return stress of the elastically-deformable diaphragms, and, because of this circumstance, this return stress may be less than the stress corresponding to the operating pressure of the damper. Furthermore, each end surface is preferably composed of a first ring, linked to the stator, and a second ring, linked to the rotor, and the diaphragm, which is ring-shaped, has inner and outer peripheral edges which are encircled by an inner and outer collar, respectively, to which the diaphragm is made unitary by vulcanization. The inner and outer collars are housed in an annular groove created by the two rings on the corresponding end surface; the water-tightness of the corresponding auxiliary chamber is obtained by means of the adhesion of the inner and outer collars to the walls of the groove, and the feeding passage between the principal chamber and the corresponding auxiliary chamber is formed by a gap created between the two rings of the end surface under consideration. The throttling of the high-viscosity fluid, for example a silicone-based fluid, is two-directional, and takes place between the end of each stator blade and the inner cylindrical, circular wall of the rotor, and means ensuring the unidirectional transfer of the fluid are, furthermore, fitted on each stator blade and are composed of an elastic plate attached by a screw on each blade which seals off an orifice drilled straight through the blade. The auxiliary chambers thus make it possible to compensate for variations in the volume of the viscous fluid filling the main chamber caused by changes in its operating temperature, because of the elastically-deformable diaphragms. These diaphragms, which are made unitary with the stator and rotor, respectively, by means of the inner and outer collars, operate under the action of torsion when motion of the rotor in relation to the stator takes place, in such a manner that they exert some degree of elastic return under the force of torsion, which tends to bring the stator and rotor back into an initial relative position. Furthermore, since the viscous fluid is introduced into the damper under pressure sufficient to produce an elastic deformation of the diaphragms which covers the fluid-volume retraction during low-temperature use, the diaphragms produce the pressurization of the volumes of viscous fluid contained in the auxiliary chambers, thereby ensuring that the main chamber of the damper will be filled under static pressure.
French Patent No. 2 573 829 describes a tubular, fluid-operated damper which produces linear, rotational damping between a tube and an axis central to the tube by means of fluid shearing between two sets of concentric, alternately telescoping tubes in a shearing chamber between the tube and the central axis. Axial water-tightness is achieved by means of two elastic connecting collars between the tube and the central axis, an arrangement such that these elastic collars also produce a rotational, linear elastic return. Furthermore, an expansion chamber, which is coaxial to the shearing chamber, is marked off between the central axis and the outer tube by an annular, expansion diaphragm which separates the expansion chamber from the shearing chamber. This expansion chamber provides for the compensation of heat-induced variation in the volume of the fluid, which is sheared in the damper. A device of this kind may be used as a resilient return strut of the hydraulic-elastic type having linear and rotational damping incorporated by shearing of fluid between two sets of concentric, alternately telescoping tubes. Furthermore, this strut is equipped with a device which compensates for thermally-produced expansion of the sheared fluid, and the elastic plugs which produce the elastic return of the central axis and the outer tube toward the original relative position, also bring about the water-tight sealing of the damper chamber. As regards their use on helicopter rotors, tubular, fluid-operated dampers of this kind do not possess sufficient rigidity, and have a complex structure which is of delicate construction, in particular because the dampers have two coaxial and alternately-telescoping sets of shearing tubes.
For these reasons, French Patents Nos. 2.528.382 and 2.592.696 propose resilient return struts of the hydraulic-elastic type having incorporated damping, which contain:
two rigid devices, each of which is equipped with hinging means designed to connect one of the rigid devices to a first part, such as a blade or a device connecting said blade to a rotor hub, and the other rigid device to a second component, such as said rotor hub,;
at least one resilient return device comprising a mass of a deformable material, which is unitary with the two rigid devices and designed to become deformed when said rigid devices are displaced one in relation to the other, and to produce on said rigid devices a resilient return action tending to bring them back to the initial relative position;
at least one hydraulic damper comprising two working chambers of volume varyIng in opposition, which contain a relatively viscous hydraulic fluid designed to circulate from one to the other of the chambers through at least one narrow circulation passage between said working chambers when said rigid devices are displaced one in relation to the other, in order to produce a damping of the relative displacement of said rigid devices; and
means for the absorption of the thermal expansion of the hydraulic fluid.
In this type of strut, also called a drag damper or frequency adapter, described in French Patent No. 2 528 382, the mass of the deformable material is a ring made of a viscoelastic material adhering by its two front surfaces to two rigid plates bolted one onto the other and onto a brace so as to constitute the rigid device which is unitary with the blade and in such a way that the brace is shaped in the form of an attachment bracket which supports a mounting ball-joint, while the second rigid device, mounted on the rotor hub by a ball-joint, has one part in the shape of an annular plate embedded in the thickness of the viscoelastic ring and around a central, oblong recess in this ring, in which a restrictive element is housed, delimiting together with the lateral wall of the central recess of the ring, the two working chambers having volumes varying in opposition. and two narrow circulation passages between the two working chambers. The restricting element is formed by the bolting of the substantially cylindrical central portions, one against the other of the two rigid parts, each of which has a circular plate housed in an opening in one of the two rigid plates of the device connected to the blade, in order to close the central hollow of the viscoelastic ring. The means for absorbing the thermal expansion of the hydraulic fluid are housed in the restricting element and are composed of two pairs of inner chambers, each of which contains a manometric capsule; the two chambers of each pair are connected to a single narrow duct which opens out at the mouth of one of the two narrow circulation passages between the working chambers.
In-flight experimentation on helicopters of medium tonnage of a main rotor comprising a new-generation hub and equipped with resilient return drag struts of the viscoelastic and hydraulicelastic type, such as those described in French Patent No. 2 528 382, has demonstrated that it was desirable to increase the damping produced by these resilient return struts, in order to properly provide the damping effect over a flight range which has been enlarged as regards weight and especially speed of which these modern helicopters, equipped with a new-generation rotor hub, are capable. However, the increase in damping applied to these resilient return struts brings with it a correlative increase in their rigidity which is incompatible with a satisfactory mechanical resistance of the means used to attach these struts to the rotor hub.
In order to correct these problems, French Patent No. 2 592 696 proposed that, in a strut of the type described above, the mass of the deformable material of the resilient return device be a cylindrical sleeve made of an elastic material, which also forms a sealing joint of the hydraulic damper, and that this sleeve be made to adhere in a water-tight manner by its inner and outer surfaces between two tubular metal cylinders which are coaxial in relation to the axis of the strut, each of these cylinders being unitary with one of the two rigid components. The hydraulic damper comprises, in addition to a piston mounted so as to slide axially in a cylinder carrying one of the rigid components and which separates one from the other the two working chambers marked off in this cylinder, a longitudinal shaft rod parallel to the axis of the strut which renders the piston unitary with the other rigid component while extending through at least one of the working chambers, as well as a compensation chamber placed between, first, a cylinder base adjacent to a working chamber through which the rod passes, and, second, a lateral surface of the cylindrical resilient sleeve and the inner lateral face of the inner tubular cylinder. This compensation chamber contains a volume of hydraulic fluid as well as a volume of gas, which constitute said means for absorption of the thermal expansion of the hydraulic fluid; in addition, it is designed to compensate for the difference between the variations in volume of the two working chambers, which results from the rod when the piston is displaced in the cylinder. Finally, the hydraulic damper contains a compensation device by which the compensation chamber is connected to each of the working chambers in order to allow for a flow of hydraulic fluid from one of the working chambers and toward the compensation chamber when the piston is displaced in one direction, and a flow of hydraulic fluid from the compensation chamber toward a working chamber when the piston is displaced in the cylinder in the other direction.
Since the sliding movement of the piston in the cylinder is accomplished in a water-tight manner, the narrow feeding passage between the two working chambers is a passage provided in the piston and in which a restriction device is installed. Furthermore, excess-pressure valves are also installed in ducts arranged through the piston in order to connect the two working chambers together, and to allow a flow of hydraulic fluid from the one of the working chambers which is pressurized, when the piston is displaced in the cylinder, toward the other working chamber which has been depressed, as soon as the differential pressure between the two working chambers exceeds a given threshold. In addition, the compensation device is housed in a compensation duct which is, itself, set in the piston, in such a way that each of its two ends are continuously connected to one of the two working chambers, and that one substantially central portion of this compensation duct is permanently connected to the compensation chamber by means of a compensation conduit extending into the piston shaft. The compensation device is a dual valve comprising two movable seals, each of which cooperates with one of two receptacles against which it is lodged in water-tight fashion by the pressure existing in one of the two working chambers, when this chamber is pressurized by displacement of the piston in the cylinder, while the other seal is drawn away from its receptacle so that the other working chamber may be connected to the compensation chamber.
An arrangement of this kind requires that the rod be guided. This function is provided by a bearing installed in a cylinder base adjacent to a working chamber equipped with mechanical packing and through which the rod passes.
The disadvantages of such a strut are, first, that its internal structure requires the use of complex valves, parts sensible to wear, and various devices which cooperate with each other while possessing slight relative play; and, second, that watertightness is obtained only with the careful installation of the sliding piston and the rod connected to this piston.